1. Technical Field
The invention relates generally to marine propulsion devices such as outboard motors and stern drive units. More particularly, the invention relates to constructions of trim cylinder/piston assemblies for marine propulsion devices.
2. Reference to Prior Art
It is generally known to pivotally connect a swivel bracket to a transom bracket for pivotal movement relative thereto about a horizontal tilt axis. It is also known to provide a trim cylinder/piston assembly which is supported by the transom bracket and which includes a rearwardly extending trim piston rod having an outer end engageable with the swivel bracket to rotate the swivel bracket about the tilt axis.
FIG. 4 in the drawings illustrates a portion of a prior art construction of a hydraulic trim cylinder/piston assembly. The prior art construction of the trim cylinder/piston assembly includes a trim cylinder A having an upper portion defining an interiorly threaded open end B. The cylinder A also includes a step-like interior surface C formed by a pair of generally annular, axially facing shoulders D, E and inner and outer coaxial cylindrical surfaces F, G located in alternating relation with the annular shoulders D, E.
The prior art trim cylinder/piston assembly also includes an end cap H having a generally cylindrical head I including an annular undersurface J which engages annular shoulder D and a peripheral surface K which is in opposed facing relation to the outer cylindrical surface F. The end cap H also includes a threaded portion L screwed into threaded engagement with the open end B of the cylinder C. The end cap H also has extending therethrough a bore M which surrounds the piston rod N.
During operation of the marine propulsion unit, the piston rod N can subject the end cap H to radial loading. The end cap H can become loosened and unseat itself by such radial loading because of the clearance between peripheral surface K and the cylindrical surface F of the cylinder A. The undersurface J of the end cap H which engages annular surface D provides a friction fit between the end cap H and the cylinder A. During operation of the propulsion unit, however, such radial loading can cause vibration of the end cap H and relative movement between the engaged surfaces D, J. Such loosening of the end cap H can compromise the integrity of the sealed hydraulic trim cylinder/piston assembly and can lead to corrosion or failure of the hydraulic trim cylinder/piston assembly.
Manufacture of the prior art construction to close tolerances can reduce the detrimental effects of such vibration, but such precise manufacture is difficult and expensive due to the critical relationship between the threaded engagement of the end cap H and the cylinder A and the clearance between the end cap's peripheral surface K and the shoulder F of cylinder A. Moreover, the manufacture of trim cylinder A to close tolerances does not eliminate occurence of loosening of the end cap H. Prior efforts to prevent loosening of the end cap H of the prior art construction have also included over-tightening of the end cap H and the trim cylinder A. Such over-tightening is undesireable from a maintanence standpoint and which also is only marginally effective. Attention is directed to the following U.S. Patents: U.S. Pat. No. 2,615,379 which issued to De Groff on Oct. 28, 1952; and U.S. Pat. No. 3,608,438 which issued to Thomas et al. on Sep. 28, 1971.